This invention relates to storage stable liquid delivery systems (LDS) for acyl isethionates. A typical acyl isethionate is sodium cocoyl isethionate (SCI) and it will be used as the reference isethionate for this invention.
Besides the SCI, two other critical components in the LDS of this invention are an imidazoline amphoteric (IA) and a second anionic surfactant (SAS) or combination of SAS's. Both are described later in this specification. The SCI is viewed as the main or primary anionic surfactant.
The LDS of this invention are suitable for use in the preparation of water based detergent formulations. For example, in personal care applications, the LDS can be letdown with water to prepare hair shampoos, hand soaps, facial cleaners, bath gels and foams, shower gels and body washes. In these formulations, the LDS are expected to impart good detergency and mildness to the skin and hair, provide good foam volume and quality and generate a creamy lather when applied to the skin or hair.
The LDS are free standing liquids at 25.degree. C. They can be made by a number of processes, but typically in this invention are made by mixing the surfactants in water at elevated temperatures until a clear solution forms and then cooling the LDS typically to 25.degree. C. and holding at this temperature. We reference here patent application Ser. No. 08/641,124, filed on Apr. 30, 1996 and assigned to Rhone-Poulenc Inc. which describes a unique process to make LDS of this invention. In that process, molten acyloxyalkane sulfonate (which is the crude reaction product of acylating an alkane sulfonic acid with a fatty acid) is quenched in an aqueous quench liquid comprised of an amphoteric and/or an anionic surfactant to produce a blend of the acyloxyalkane sulfonate and amphoteric and/or anionic surfactant. To the extent necessary for completion, this application is expressly incorporated by reference.
In order to deliver SCI in a water-based LDS, one must overcome the major physical property obstacle of SCI: its very limited water solubility. At 25.degree. C., it is soluble at 0.01% by weight. It is surprising that one can make SCI based LDS that can contain SCI concentrations that are over 1000 times its water solubility at 25.degree. C.
The LDS can be clear, translucent, or opaque liquids at 25.degree. C. The appearance is primarily dependent on the concentration of SCI and the ratio of SCI to other contained surfactants; the pH of the system can also play a role. At 25.degree. C., the LDS are flowable liquids that are pourable and pumpable in many cases. At 25.degree. C., these flowable LDS have viscosities typically at or below 30000 cps.
At elevated temperatures (35-50.degree. C.), the LDS remain as flowable liquids. If they are opaque at 25.degree. C., they can become clear or translucent systems between 35-50.degree. C. No precipitation of particulate solids or loss of clarity occurs in the LDS when exposed both to elevated temperatures and a broad pH range (6.0-9.0). At high temperatures and pH above 8.0, SCI can potentially become hydrolytically unstable and one would expect to see precipitation of solid matter, loss of clarity and/or gel formation. None of these effects occur in the inventive LDS after typical exposure periods. By reference, please compare the article by P. J. Petter, "Fatty Acid sulphoalkylamides and Esters as Cosmetic Surfactants," International J. of Cosmetic Science, 6, 249-260 (1984).
Typically the inventive LDS are maintained at pHs equal to 7 or greater, more typically from pH=7.5-8.5, and most typically for pH=7.8-8.2. In the most typical pH range, the SCI is hydrolytically stable, the outstanding hydrotroping and solubilizing power of the IA is maintained, and the LDS viscosity is such that they are pourable and in many cases pumpable systems and are typically storage stable over a wide temperature regime. Of course, pH's at or below 7.0 can be used, but the overall beneficial properties of our LDS are best exhibited at pHs above 7.0.
At temperatures below 25.degree. C., e.g. 4-22.degree. C., the LDS of this invention containing low to moderate amounts of SCI typically are opaque, flowable liquids, but LDS with high SCI contents are typically opaque, nonflowable creams, pastes or gels. These nonflowable systems can become flowable again by heating them to slightly elevated temperatures (30-45.degree. C.); in many cases these nonflowable, opaque systems will not only become flowable at these elevated temperatures, but clear as well.
Two component LDS that contain an SCI and an IA or a betaine can be made but these suffer from a number of technical limitations. Rhone-Poulenc's application Ser. No. 08/641,124, filed on Apr. 30, 1996 describes in part two component systems. In this application, one can see that at ambient conditions, these two component systems are typically pastes or very viscous liquids that appear to require relatively high loadings of the IA or betaine to effect SCI solubility. It would be impractical to work with these very viscous liquids and pastes in making personal care or other detergent formulations. Easily flowable, two component systems would either have a commercially unattractively low total active content (&lt;25%) or would have too low of an IA or betaine content that could solubilize SCI concentrations that would be commercially attractive (.gtoreq.12%).
In addition, solubilizing the SCI by the IA or betaine in liquid two component systems requires a relatively high weight ratio of IA or betaine to SCI, typically greater than one. For example, in personal care formulation where the SCI to IA or betaine actives ratio needs to be equal to or greater than one, additional SCI would have to be added and heated to high temperatures (70-90.degree. C.) to dissolve it; this heating step defeats the purpose of creating LDS, which should not require or require only modest heating (30-40.degree. C.) when formulating personal care formulations.
Overall, two component LDS are either impractical to handle; or if handleable, would be expected to have too low an active concentration for commercial interest; or unstable in that useful amounts of SCI in the two component LDS would precipitate at ambient conditions because of insufficient IA or betaine content.
In this invention, surprisingly, we have found that replacing some of the IA with an SAS or adding an SAS to a two component LDS largely overcomes the above limitations.
LDS containing significant quantities of organic surfactant that have limited water solubility at room temperature such as SCI are relatively rare, but do exist. For example, they are sold under the tradename MIRASHEEN licensed to Rhone-Poulenc, Inc. and EUPERLAN licensed to Henkel. In these LDS, ethylene glycol mono and/or distearate, which are essentially insoluble in water at 25.degree. C., are dispersed in a stable form in water at room temperatures by the presence of suitable emulsifying type surfactants. These LDS are opaque at ambient conditions. By reference, U.S. Pat. Nos. 5,529,721, 4,777,038 and 5,560,873 are included as recent examples.
By contrast, no comparable LDS containing the compositions of this invention are known to be manufactured and sold under a commercial tradename.
We are aware that formulated liquid personal care products may contain a combination of surfactants that are covered in this invention. However, these formulated products may use any one of them, e.g. SCI, at very low actives concentration and the total surfactant active concentration is generally below the lower limit of the actives content of this invention. By reference, U.S. Pat. No. 5,518,647 (Colgate) and EP692240 (L'Oreal) are included as examples.
Formulators of personal care and other household detergent products will find many advantages in using LDS. First, they enhance formulation quality through consistent performance of the surfactant mixture; second, they reduce batch adjustment and cycle times to make the formulations; third, they provide cold or low heat mixing opportunities thereby reducing and/or eliminating high energy heating processes; fourth, they allow for increased plant throughput and overall reduction in energy and manpower costs; fifth, quality control time is reduced with the elimination of many individual surfactants to purchase and monitor; sixth, handling costs through consolidated warehousing, bulk storage, and elimination of partial drums are decreased; and seventh, most solids and viscous liquids are eliminated. In addition for SCI based LDS, the irritancy of SCI dust particles is eliminated as a potential health hazard.
Some literature relates to surfactant mixtures containing SCI. U.S. Pat. No. 5,415,810 relates to the use of zwitterionic surfactants such as simple betaines, amidobetaines, and sulphobetaines and is not directed toward the use of an IA; in fact, no direct mention of an IA can be found. In addition, U.S. Pat. No. 5,415,810 does not direct itself toward stable, free standing surfactant mixtures but instead immediately incorporates the mixtures into a personal care product where the SCI is stabilized by a variety of well-known additives used in personal care products and where the SCI and other surfactants are diluted in water for enhanced solubility.
In U.S. Pat. No. 4,243,549 are described highly concentrated mixtures of anionic and imidazoline amphoterics among other mixtures. However, U.S. Pat. No. 5,243,549 is directed toward concentrated mixtures whose surfactant actives are essentially above 60% by weight and the mixture is predominantly in the lamellar or "G" phase. In addition, U.S. Pat. No. 4,243,549 directs itself toward anionic surfactants such as alkyl ether sulfates which are soluble in substantial amounts in water and is not directed toward SCI which are marginally soluble in water at ambient conditions. LDS of this invention have surfactant actives of 50% by weight or less and are predominantly if not completely outside of the "G" phase.
Other literature exists which show the utility of SCI in combination with an IA and a second anionic surfactant (SAS). However, the products described therein are outside the scope of this invention because the total surfactant actives is less than that of this invention; the percent SCI or IA is less than that of this invention; the percent of SCI plus IA is less than that of this invention; the SAS is greater in concentration than that of this invention and/or the products described are solids, not liquids. Some patents as previously referenced are U.S. Pat. Nos., 5,372,751, 5,518,647 and EP692240.